"écoute" is the verb form that can be used with any of those pronouns - he, she, or it. In this case, the pronoun is "Elle", so we know it's "she".

Also, "listens to" needs an object. You can listen to the radio, or to your mother, but since in this case the French doesn't say what she listens to, then there is no place for the "to" and it is simply "she listens".